Review Defining Division of Labor in Microbial Communities Samir Giri 1, Silvio Waschina 2, Christoph Kaleta 2 and Christian Kost 1 1 - Department of Ecology, School of Biology/Chemistry, University of Osnabrück, Osnabrück, Germany 2 - Research Group Medical Systems Biology, Institute for Experimental Medicine, Christian-Albrechts-University Kiel, Kiel, Germany Correspondence to Christian Kost: [email protected] https://doi.org/10.1016/j.jmb.2019.06.023 Abstract In order to survive and reproduce, organisms must perform a multitude of tasks. However, trade-offs limit their ability to allocate energy and resources to all of these different processes. One strategy to solve this problem is to specialize in some traits and team up with other organisms that can help by providing additional, complementary functions. By reciprocally exchanging metabolites and/or services in this way, both parties benefit from the interaction. This phenomenon, which has been termed functional specialization or division of labor, is very common in nature and exists on all levels of biological organization. Also, microorganisms have evolved different types of synergistic interactions. However, very often, it remains unclear whether or not a given example represents a true case of division of labor. Here we aim at filling this gap by providing a list of criteria that clearly define division of labor in microbial communities. Furthermore, we propose a set of diagnostic experiments to verify whether a given interaction fulfills these conditions. In contrast to the common use of the term, our analysis reveals that both intraspecific and interspecific interactions meet the criteria defining division of labor. Moreover, our analysis identified non-cooperators of intraspecific public goods interactions as growth specialists that divide labor with conspecific producers, rather than being social parasites. By providing a conceptual toolkit, our work will help to unambiguously identify cases of division of labor and stimulate more detailed investigations of this important and widespread type of inter-microbial interaction. © 2019 Elsevier Ltd. All rights reserved. Introduction executed. Due to the tremendous advantages that can result from dividing tasks among several, lower In order to survive and reproduce, organisms level units, the principle of DOL can be found at all need to accomplish many different tasks such as the levels of biological organization [6–8]. acquisition of food, defense against enemies, Enzymes that can catalyze two biochemical growth, repair, and so on. In modular organisms reactions with different efficiencies can—after a that consist of multiple, lower-level units, different duplication event of the underlying gene—be select- tasks are executed by specific modules that are ed to perform both functions with an increased specialized for their respective task, a phenomenon specificity [9,10]. Also, the evolutionary success of that is called division of labor (DOL) [1–3] or multicellular organisms is likely due to the differen- functional specialization [4,5]. Breaking down com- tiation into several cell types or tissues that all serve plex processes into simpler steps eliminates unnec- specific functions and synergistically interact with essary constraints that stem from the need to each other to enhance the performance of the whole perform several tasks simultaneously or switch organism. The prime example for DOL, however, is between them, thus significantly enhancing the eusociality, which evolved multiple times indepen- efficiency, with which the whole process can be dently in the animal kingdom [11]. Analogous to the 0022-2836/© 2019 Elsevier Ltd. All rights reserved. Journal of Molecular Biology (2019) 431, 4712–4731 Review: Division of Labor in Microbial Communities 4713 or termites [12,13]. These insect societies divide labor at two levels of organization: first, reproductive DOL in the form of a single (or multiple) fertile queen (s) that exclusively focus on the reproduction of a given colony, while sterile workers perform all tasks related to colony maintenance and growth. This dichotomy is functionally equivalent to the distinction of germline and soma [14],asitiscommonly observed in multicellular eukaryotes [15,16]. Sec- ond, sterile workers within a eusocial colony are differentiated into castes that are defined by different sizes and ages, and which fulfill special functions within the colony. This type of DOL is analogous to the abovementioned cells and tissues of multicellular organisms that fulfill certain tasks that all contribute to the growth and maintenance of the whole organisms [13,17]. Finally, also bacteria commonly show DOL, such as, for example, the development of fruiting bodies in Myxococcus xanthus [18]. Formation of this complex structure requires the contribution of multiple cells performing different tasks, including cell lysis, the formation of peripheral rods, or the development into spore cells. Spores and lysed cells mainly exist inside of fruiting bodies, whereas peripheral rods remain outside. Peripheral rod cells have been proposed to enhance the survival of Myxococcus in its natural habitat, because they do not undergo cell division, but can probably respond to sudden change in environmental conditions [19]. Furthermore, lysed Fig. 1. Trade-offs can cause DOL. (a) Consider a cells provide nutrients that allow spore cells to situation in which an organism needs to be able to perform two different tasks (A + B) in order to maximize its fitness. If differentiate during development. the organisms can perform both tasks simultaneously Another prominent examples of DOL within the equally well, it is considered to be a generalist. A trade-off bacterial kingdom are filamentous cyanobacteria occurs when an improved performance of one task comes at such as Anabaena or Nostoc species that differen- the expense of a reduced ability to perform the respective tiate into specialized cell types to segregate incom- other task. Under these conditions, organisms need to patible biochemical reactions such as specialize and are thus only able to perform either one of the photosynthesis and nitrogen fixation. Photosynthe- two tasks (A or B) or display reduced abilities to perform both sis occurs in vegetative cells and the adjacent tasks simultaneously. The shape of the dashed lines heterocysts fix nitrogen, which requires a highly connecting the two specialist strategies (i.e., pareto fronts) regulated system to ensure efficient distribution of dictates how stringent (blue background) or relaxed (yellow background) a certain trade-off is. (b) A synergistic resources along the filament [20,21]. interaction between two specialists (A and B) can enhance Besides the abovementioned cases of bacterial fitness of the entire consortium. The graph shows the fitness DOL, there are many other examples of behaviors both types reach under mono- (left) and coculture conditions that appear to be cooperative or beneficial at the (right). When interacting with each other, both types can level of a bacterial consortium. However, given the display several different consortium-level growth pheno- paradigmatic cases mentioned above, it is frequently types: (I) fitness of both partners combines additively, (II) not clear whether or not these other cases constitute one partner benefits, while the other displays a reduced real examples of DOL, or rather some different type fitness relative to monoculture conditions, and (III) both of ecological interaction. One example for such an partners benefit (i.e., their fitness is enhanced compared to ambiguous case is the biodegradation of a complex monoculture conditions). DOL can be seen in consortia of substrate such as a polysaccharide (e.g., hemicel- type III and intraspecific consortia of type II (see main text for – further explanation). lulose [22 24]) or xenobiotics (e.g., organophos- phate esters and pentachlorophenol [25–29]). In these cases, degradation of a single molecule previous examples, also in this case, DOL provides requires many sequential biochemical reactions. benefits at the level of the social colony and is However, most commonly, it is not a single species thought to be one of the main factors responsible for that breaks down the entire substrate all alone, but the tremendous ecological success of eusocial ants multiple species that work together to achieve this 4714 Review: Division of Labor in Microbial Communities goal. In these cases, every species catalyzes an can manifest not only among members of the same individual step of the degradation pathway and species, but also, in cases where the trade-off is releases an intermediate metabolite, which is then evolutionarily conserved, between different species passed on and used by the next species in the chain. [35–37]. In any case, the ability of individuals to Such sequential interactions help to not only perform both functions simultaneously is determined efficiently utilize the available resources but also to by the shape of the trade-off function (Fig. 1a). remove toxic intermediates that often inhibit the Specifically, when the trade-off is rather stringent, growth of other community members. In this pro- the shape of the trade-off is concave, while convex cess, different bacterial species come together and relationships point to more relaxed trade-offs (Fig. 1a). collectively degrade a substrate much more effi- Three extreme ends of this distribution are (i) the point ciently than any of the